In orthogonal frequency division multiplexing (OFDM) communication systems, the guard interval (GI) length is fixed and defined such that it covers the longest multipath delay encountered in a worst-case scenario, that is, large cell size, long range and reflection at distant objects. In more advanced standards such as Digital Video Broadcasting-Terrestrial (DVB-T) according to ETSI ETS 300744, V1.5.1, 2004 or Long Term Evolution (LTE) according to ETSI TS 136211, version 11.0.0 Release 11, October 2012, different GI lengths are defined, e.g., the three GI lengths associated with LTE formats that are: normal cyclic prefix (CP) according to 4.69 μs for the majority of macro cells, extended CP according to 16.67 μs for large cells and small single-frequency networks, and another extended CP according to 33.33 μs for large SFN. These GI lengths, however, are chosen by the operator based on cell planning and are fixed during operation.
A major issue with conventional CP-OFDM is the missing flexibility of the guard interval in terms of length and content. During operation, the GI cannot be tailored to the prevalent channel dispersion. Therefore, the GI length is usually chosen conservatively to cover the worst-case channel dispersion expected. Under typical, more benevolent channel conditions, this entails a substantial loss of transmission capacity and throughput, a disadvantage which also translates into multi-user environments, i.e., the GI length cannot be tailored to individual users.
Another issue with conventional CP-OFDM is the interdependence between the GI length and the frame format in which OFDM symbols are embedded. A change in GI length (if possible at all) necessarily entails a change in the frame structure, i.e., the number of OFDM symbols in a frame. For instance, if it were possible to switch between the three LTE CP lengths during operation, the frame format would have to change also. As there are very few possibilities of casting variable-length OFDM symbols into a frame of fixed length, GI flexibility is severely limited in single-user scenarios and impossible to achieve in multi-user environments.